1. Field of the Invention
The present invention relates to a rubber composition which is highly adhered to a metallic material by a vulcanization. Heretofore to promote adhesion between a rubber and a metallic material such as a steel cord, it has been known to plate the surface of the metallic material of steel or steel cord with a copper alloy such as brass and bronze. However, the need has increased for high speed, high load and high efficiency of motor car tires and conveyor belts, and driving belts, V-belts, hoses, etc., wherein metallic materials are used as tension members. Accordingly, a high adhesive bond between the rubber and the metallic material is needed. Recently, in order to improve the adhesive bond between a rubber and a metallic material, it has been proposed to add a mixture of a formaldehyde donor, a formaldehyde acceptor and a finely dispersed silica filler (it is called "HRH" system) to the rubber. However, rubber compositions including the HRH system, such as a mixture of fine silica, resorcinol and hexamethylenetetramine, have the disadvantages of generating a smog and bad smell to pollute environment and unsatisfactory workability when blended by a Bambury's mixer or a roller mixer because of the severe sublimation of resorcinol. The rubber compositions have also the disadvantage of unsatisfactory rolling workability because of low scorch safety and the disadvantage of deteriorated adhesive property caused by moisture absorption in the unvulcanized condition.
It has been proposed to add cobalt naphthenate in order to improve the adhesive property between a rubber and a metallic material. However, in said case, there is the disadvantage of deterioration of adhesive property in high temperature blending operation or high temperature vulcanization; (such as 150.degree. - 190.degree. C in the preparation) and also the disadvantage of deterioration of adhesive property and deterioration of other physical properties after heat aging; (such as about 100.degree. C for several tens of hours of driving).
2. Summary of the Invention
It is an object of the present invention to improve the adhesive property between a rubber and a metal metallic material under vulcanization. It is another object of the invention to limit the deterioration of the adhesive bond caused by heat aging and to improve workability at the same time in the preparation of rubber products such as tires, belts and hoses. The objects of the invention have been attained by providing a rubber composition which comprises 0.5 - 20 wt. parts of a metal salt of hydroxybenzoic acid per 100 wt. parts of a rubber. (phr. : weights per hundred rubber).